Solar Max carries an array of instruments to study sun's activity

The Solar Maximum Mission satellite is the world's only orbiting solar observatory. It was launched Feb. 14, 1980, primarily to study solar flares. These are violently explosive events on the sun that send electromagnetic radiation and particles into space. Such outbursts often disrupt communications on Earth and even create voltage surges along power lines. They are most frequent as the sun's activity cycle reaches a maximum once in roughly every 11 years.

Solar Max has six instruments that monitor flare radiation through a spectrum of frequencies ranging from that of visible light through the ultraviolet to the extremely high frequencies of soft (lower frequency) X-rays, hard (higher frequency) X-rays, and gamma rays. An instrument that measures a range of frequencies is usually called a spectrometer.

Instruments that measure polarization of light and other electromagnetic radiation are generally called polarimeters. Radiation is said to be polarized when its electric component vibrates only in one plane.

Instruments that measure the temperature and density of material in a flare may be called polychromators because they essentially look at the ''colors'' in the flare's radiation.

Solar Max has an Ultraviolet Spectrometer/ Polarimeter, an X-Ray Polychromator, a Hard X-Ray Spectrometer, and a Coronograph/Polarimeter, the electronic box of which has malfunctioned and is to be replaced.

These four instruments lost their capability during the 10th month after launch when the Solar Max attitude control module had blown three fuses and the spacecraft could no longer maintain precision pointing. If astronauts replace that module, this capability will be restored.

There are three other instruments that have continued to function, since they work when the spacecraft is oriented only roughly toward the sun.

These are a Gamma Ray Spectrometer, a Hard X-Ray Burst Spectrometer, and a so-called Active Cavity Radiometer/Polarimeter.

The latter instrument measures the sun's total radiation, a matter of intense interest because small changes in this total radiation may affect Earth's weather and climate.

The sun now is in a relatively quiet period, so if the Challenger astronauts can repair the Solar Max satellite and replace it on orbit, this valuable observatory should be able to observe the sun as its activity rises toward a new peak.

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